The present invention relates to nanosensors, in particular nanosensors for biological, medical and environmental applications, and more specifically a three-dimensional nanoresonator structure.
The invention also relates to a method for the manufacture, by lithographic techniques, of three-dimensional nanoresonators that are dispersible in a liquid such as water.
Nanoresonators, also known as nanoantennas, are resonator devices of nanometric dimensions which, when exposed to wide-spectrum exciting electromagnetic radiation, show increased absorption at a natural resonant frequency determined by the characteristics of the resonator structure and/or by the interactions with the environment in which they are immersed, this frequency being located between the THz range and the near infrared wavelengths.
Two classes of nanoresonators, namely structured nanoresonators and nanoparticles with resonant properties, are known in the prior art.
Structured nanoresonators may be described as nanometric-scale circuits, the resonant properties of which are determined by the inductance and capacitance characteristics of the circuit, and are therefore also known as LC metamaterials.
Metamaterials have characteristic properties which depend on the geometry of the device, instead of on its chemical composition.
The most common structured nanoresonators are based on a two-dimensional open-loop configuration, such as that shown in FIG. 1 together with an equivalent electrical circuit.
This two-dimensional nanoresonator structure comprises a conductive metal microstrip (typically made of gold) shaped to form an open loop, in which, at the operating frequencies of the device, the loop-shaped path shows distributed inductive behaviour and the gap in the ring acts as a capacitor. The corresponding values of capacitance and inductance depend on the geometrical parameters of the microstrip, and the resonant frequency f=1/√LC can easily be tuned by modifying the dimensional parameters of the circuit during the design process.
Nanoresonators of this type have an intrinsically two-dimensional structure, which has to be supported on the surface of a substrate from which, therefore, they cannot be separated. There is a known method in the prior art for making arrays of nanoresonators anchored to a substrate by using a lithographic procedure for the versatile configuration of at least one layer of material deposited on the substrate by planar deposition techniques.
The resonance of these devices can be tuned over a wide range of wavelengths, by suitable design of the resonance structure, but their use is limited by the dimensions of the array and by the nature of the substrate to which they are anchored.
WO 2011/050272 describes an array of two-dimensional nanoantennas and processes for the efficient manufacture of an array of nanoantennas whose shape is controlled by nanostencil lithography. This method can be used to produce nanoresonator structures on virtually any type of support, whether conductive, non-conductive or magnetic, with properties of flexibility and stretchability if required. The array of nanoantennas formed in this way can be used in spectrometry, for the detection of bioanalytes or inorganic chemical substances having resonant frequencies in the infrared range.
The paper by M. Nagel, F. Richter, P. Haring-Bolivar and H. Kurz, entitled “A functionalized THz sensor for marker-free DNA analysis”, published in Phys. Med. Biol., 2003, 403625-3636, describes a functionalized biochip for conducting DNA hybridization experiments. The circuit includes an array of resonators operating at THz frequencies, each comprising a first metal electrode anchored to a substrate, an intermediate non-conductive polymer layer, and a second conductive metal electrode for functionalization, adapted to bind DNA strands. These resonators are used to detect the presence of molecules which bind to the functionalized surface, making use of the fact that their resonance properties vary as a function of the presence of these molecular bonds.
These devices are typically used for in vitro biological analysis, in analyte assays for example, but have the disadvantage that they cannot be injected into a living organism and traced in vivo, because they cannot be separated from the substrate.
Unlike structured nanoresonators, some free metallic nanoparticles, which can be produced by chemical synthesis, are known to act as nanoresonators because of the resonance of the plasmon waves which are established on the surfaces of the molecules.
For example, nanoparticles of Ag:SiO2 and Au:SiO2 in colloidal suspension have been used as electromagnetic nanoresonators, as described by A. Kudelski and S. Wojtysiak, in “Silica-Covered Silver and Gold Nanoresonators for Raman Analysis of Surfaces of Various Materials”, published in J. Phys. Chem. C. 2012, 116 (30), pages 16167-16174.
As a general rule, resonant nanoparticles, not bound to any substrate, can be dispersed freely in a fluid medium and can be used advantageously for in vivo applications, by exploiting, where appropriate, their capacity to bind to molecular species present in the fluid medium, which affect their resonance properties.
Unfortunately, however, the resonance properties are mainly determined by the intrinsic characteristics of the material of the nanoparticles, and since the chemical synthesis process does not enable complexes of nanoparticles to be produced with controlled shapes, the resonance can only be tuned over a limited range of wavelengths.
K. H. Su et al., in “Tunable and augmented plasmon resonances of Au/SiO2/Au nanodisks”, published in Applied Physics Letters, vol. 88, no. 6, 10 Feb. 2006, describe three-dimensional resonant nanostructures (nanodiscs) anchored to the original substrate, manufactured by a top-down procedure based on an EBL (Electron Beam Lithography) process on a quartz substrate, on which a first conductive gold layer, an intermediate dielectric layer of SiO2 and a second conductive gold layer are deposited in succession by evaporation. A standard lift-off process defines the three-dimensional structure of the nanodiscs without their material separation from the substrate.
D. J. Wu et al., in “Tunable near-infrared optical properties of three-layered gold-silica-gold nanoparticles”, published in Applied Physics B, vol. 97, no. 1, 3 Mar. 2009, provide a theoretical description of plasmon resonance in nanospheres with layers of gold, silica and gold.
Bhuwan Joshi et al., in “Numerical Studies of Metal-Dielectric-Metal Nanoantennas”, published in IEEE Transactions on Nanotechnology, vol. 9, no. 6, 6 Nov. 2010, present a theoretical study of cubic and cylindrical nanoantennas formed with two conductive layers separated by an intermediate dielectric layer.